This invention relates to missile navigation, and more particularly to an improvement in the Synthetic Array Radar Command Air Launched Missile.
The Synthetic Array Radar Command Air Launched Missile (SARCALM), as its name implies, is designed to deliver a missile to a ground target located on a synthetic array radar map. The synthetic array radar consists of a coherent transmitter and receiver, a synthetic array processor and display, and a velocity sensor and motion compensation whose function is to focus the map for the length of a synthetic array. The target is located on the map in the time-frequency coordinates. SARCALM guides the missile to a point whose time-frequency coordinates match those of the target.
Guiding the missile to match the time coordinates of the target is straightforward. Guiding the missile to the doppler coordinate of the target cannot be done directly. This is because the missile is in motion with respect to the target. SARCALM overcomes this problem by means of a coordinate transformation. The direction cosine from an interferometer antenna to the missile is readily measured. The direction cosine from the antenna to the target can be derived from its doppler frequency. Thus, guidance in the frequency coordinate is replaced by guidance in the direction cosine coordinate.
An iso-range surface is a sphere; an iso-doppler surface is a cone. The interferometer antenna boresight plane matches the iso-doppler surface at the vicinity of the target. All surfaces are centered at the radar antenna. A missile launched from the aircraft flies at a constant altitude in the direction of the boresight plane. Upon approaching the target's range the missile starts to dive. A two-axis direction finding antenna on the missile points the missile yaw axis in the direction of the radar. An inertial sensor points the roll axis of the missile in the direction of local vertical. As a result, pitch maneuvers correct the range while yaw maneuvers correct the direction cosine. The intersection of a sphere and a plane is a circle. The missile is guided along this circular course until it collides with the target. Thus, what is inherently a three-dimensional guidance problem is solved in the two dimensions of the synthetic array radar.
Theoretically, the translation of the doppler frequency of the target to its direction cosine from the interferometer antenna can be done by direct computations. This, however, requires that pertinent parameters be known to an accuracy commensurate with the required guidance precision. Thus, the velocity vector V needs to be known to an accuracy commensurate with the resolution of the synthetic array radar. Furthermore, the mechanical direction of the antenna boresight plane needs to be known to the same accuracy and the electrical boresight needs to coincide with it.
Satisfying the necessary conditions for pointing by computation is at present impractical. Therefore, SARCALM resorts to pointing by angle tracking the radar returns from the target. These returns are identified with information generated by the synthetic array map. In synthetic array mapping the velocity vector is used to focus the map in the time-frequency domain for the duration of an array time. This can be done with a relatively low accuracy sensor provided it can precisely sense velocity changes. Furthermore, with target angle tracking it is not necessary to calibrate the direction of the antenna boresight axis either absolutely or in relation to the electrical null. This is because both the target and the missile signals undergo the same distortions in the antenna and have the same electrical null.
The pointing oscillator designates the direction of the target. Tracking the returns from this direction with the interferometer points the electrical null at the target. In this case the doppler angle error is proportional to the angle between the azimuth null axis and the target azimuth.
A null command generator, as its name implies, senses the doppler angle error and drives it to zero by pointing the electrical null to the target azimuth. The pointing accuracy is a function of the signal to noise ratio. The signal to noise ratio is enhanced by estimating the doppler angle error from the total clutter rather than just the target return. The best estimate is obtained by a process which aggregates the individual estimate of each independent scatterer on the terrain.